The present invention relates to high speed printers having moving heads or carriages which carry printing fonts such as rotating daisy wheels. More particularly, the invention relates to apparatus utilized for adjusting such printers to accept a variety of paper thicknesses such as by making one or more carbon copies.
In the prior art, it has been recognized that with an impact printer, for example, a daisy wheel printer, wherein the printing elements are impacted by a hammer mechanism, in order to provide uniform print quality with varying paper thicknesses, it is necessary to adjust the platen location so that the surface of the paper to be printed remains a predetermined distance from the daisy wheel. Thus, it has been recognized that, without such an adjustment, thick forms placed in the printing mechanism can bind against the daisy wheel and destroy this mechanism which rotates at extremely high speed. In addition, the hammer mechanism is designed to operate optimally with a given reciprocating distance, which distance would change with varying paper thicknesses if no adjustment were included in the printer.
In order to accommodate such varying thicknesses of paper stock or varying numbers of copies, the prior art has typically mounted the platen and its associated roller assemblies on a movable sub-frame which could be adjusted on the main printer frame. This, of course, requires an adjustment at both ends of the platen to a uniform extent, which in turn typically requires relatively complicated linkages.
In addition, the requirement that the entire platen be moved, requires special guideways for this sub-frame to assure that the platen remains rigid in each of its adjusted positions. This adjustment technique adds substantial complication to the mechanics of a high speed printer, and thus adds to its cost.
It is believed that the alternative approach disclosed in the present patent application, which provides an adjustment of the carriage mechanism rather than the platen, has been avoided in the prior art because it is generally assumed that the carriage mechanism, which must move rapidly under servo motor control, requires firmly mounted guide rods to provide repeatable positioning. Thus, the prior art has uniformly adjusted the platen position, leaving the carriage mounting assembly and, in particular, the carriage guide rods, as a rigid portion of the printer frame to assure accurate tracking of the printer carriage.